A steam turbine is used to drive a machine or the like and includes a turbine body having a rotor which is rotatably supported. Steam is supplied to the turbine body, and thus, the rotor is rotated.
In the steam turbine, when an operation thereof is stopped, problems such as the rotor being bent due to unbalanced heat or being deflected by its own weight may occur. Accordingly, in the steam turbine, a turning device which rotates the rotor at a low speed when the operation of the steam turbine is stopped is used (for example, refer to Patent Document 1).
Meanwhile, there is a steam turbine having an extraction line which extracts steam inside a turbine body from an intermediate stage. In the steam turbine, the extracted steam is supplied to a preceding stage or supplied to other facilities. However, since the steam turbine is stopped and a pressure inside the turbine body decreases, the extracted steam may flow back to the inside of the turbine body. Therefore, in the extraction line, a valve device for preventing a backflow of the extracted steam is provided.
In the valve device, the valve body can be opened and closed by being driven by an oil cylinder which moves a piston by control oil. In a case where the steam turbine is continuously operated over a long period of time, the valve device remains open to continue the extraction of the steam. As a result, since the position of the valve body does not move, the position of the piston which is configured to drive the valve body is not changed, there is a possibility that the piston may become stuck inside the oil cylinder and may become immovable.